The physical layer of a wireless local area network supports a link adaptation technology. The main procedure of the link adaptation technology is: After a transmitter sends data to a user by using a certain modulation and coding scheme (Modulation and Coding Scheme, hereinafter briefly referred to as MCS), due to the impact of channel conditions, and because a receiver can estimate actual channel state information, the receiver calculates parameters such as parameters related to the channel conditions and the recommended MCS according to the channel conditions and transmission conditions (for example, the MCS) after receiving the data, and feeds back the parameters to the transmitter. After obtaining the parameters fed back by the receiver, the transmitter adjusts and selects a proper modulation and coding scheme to reduce the bit error rate of data transmission. The process of adjusting the MCS of the transmitter through modulation and coding scheme feedback (Modulation and Coding Scheme Feedback, hereinafter briefly referred to as MFB) of the receiver is a typical representative of the link adaptation technology.
A conventional wireless local area network system supports two feedback mechanisms, i.e., passive feedback and active feedback. Passive feedback means that after the transmitter sends a modulation and coding scheme feedback request (MFB Request, hereinafter briefly referred to as MRQ) to the receiver, the receiver immediately provides MFB with respect to the data of the previous frame sent by the transmitter; active feedback means that the receiver actively provides MFB, without requiring the transmitter to send a request. The active feedback may relate to the data of any one frame or multiple frames sent by the transmitter previously.
The active feedback differs from the passive feedback mainly in that the active feedback does not require the transmitter to send a request. The benefit of the active feedback is that the receiver may actively provide MFB according to actual conditions, thereby having higher flexibility. However, the active feedback has the following disadvantage: The transmitter is unable to know the data of which frame sent by the transmitter is related to the adaptation feedback of the receiver, because the receiver may provide feedback with respect to the data of any one frame sent by the transmitter previously. Therefore, the transmitter needs to know the transmission conditions used when the transmitter sends the data previously, e.g., a coding type (Coding Type), a group identifier (Group Identifier, hereinafter briefly referred to as “Group ID”), and whether to use beamforming.
However, in the conventional active link adaptation feedback mechanism, feedback information is not complete, which leads to a poor effect of link adaptation.